Power generation stations located remote from a load require transmission systems capable of efficiently transmitting the generated power from the power generation station to the load. Such systems include but are not limited to, for example, a remote wind turbine generator located in the sea, and a power grid located on land. Often a transmission cable of the transmission system must be located either submarine or subterranean in order to efficiently span a distance between the two. In addition, often the power generation station produces alternating current (“AC”) electrical power, and often the load requires AC power. However, the transmission of AC power over long distances can be problematic.
In addition to resistive losses present in the transmission of both AC and direct current (“DC”) electric power, the transmission of AC power generates reactive current resulting from a capacitance of the transmission cable and the cable consumes reactive losses from an inductance of the transmission cable. Increasing voltage and, in turn, decreasing current in AC power transmission can reduce resistive and reactive losses, which are proportional to a square of the current in the cable. However, capacitive charging current, which is a function of the voltage, frequency, cable geometry and insulation medium, may remain high, generally increasing with voltage.
The capacitance, C, per unit length of a transmission cable is determined by the geometry of the transmission cable and the dielectric constant(s) of the insulation surrounding the transmission cable. Charging the cable capacitance requires a capacitive charging current (“icc”). The capacitive charging current per phase may be given roughly by the equation:icc=Vline-neutral*ω*(Lcable*Ccable)where V is the line-neutral voltage, ω is the electrical frequency in radians per second (ω=2πf; 60 Hz=377 rad/s; 50 Hz=314 rad/s), L is the transmission cable length in kilometers, and C is a transmission cable capacitance in farads per km. It can be seen that for a given transmission cable, the conventional practice of increasing the voltage V in order to overcome resistive line losses and inductive losses generally has the effect of increasing the charging current icc, particularly in consideration of the increase in voltage and the fact that higher voltage generally necessitates thicker insulation, further increasing the capacitance.
It can be seen that for a given cable carrying a given electrical power, as the length L of the cable increases so does the charging current icc. Since a given transmission cable has a maximum current carrying capacity, any charging current icc carried by the transmission cable to accommodate the capacitance of the cable directly reduces the amount of current the cable can deliver to the load. As a result of the charging current icc of submarine and subterranean cables, conventional practice limits the transmission of AC electric power at frequencies of 50 Hz-60 Hz to distances of not more than approximately 50 km. At or below this length, the transmission cable is capable of delivering AC electric power with few operational constraints.
When transmission distances exceed 50 km, the AC electric power is conventionally converted into direct current (“DC”) electric power. Transmission of DC electric power does not suffer from the reactive losses found in the transmission of AC electric power. However, in order to transmit DC power from a power generation station that produces AC power to a load that operates on AC power, generated AC power must be converted to DC power, and received DC power must be converted back to AC power for the load. Converting generated AC power into DC power requires an expensive AC to DC power transmission terminal to be installed at the power generation station (sending end), and an expensive DC to AC power transmission terminal to be installed at a receiving end, prior to the load. Additionally, there are few vendors of high voltage DC cable and submarine DC power transmission has unique operating and maintenance practices and specialized engineering, which can result in high cost of design, operation and maintenance. Consequently, there remains room in the art for improved power generation and transmission.